A key mechanism for immune evasion and persistent infection by the Lyme disease spirochete, Borrelia burgdorferi, is antigenic variation of the VlsE surface protein. Despite the presence of a substantial number of additional proteins residing on the bacterial surface, VlsE is the only antigen that exhibits ongoing variation of its surface epitope. Recent work in our lab has identified a possible VlsE-mediated immune evasion system that allows non-VlsE surface antigens to escape the killing effects of host antibodies. Despite this recent evidence, a role for VlsE in modulating the host immune system has never been explored to date. Moreover, studies investigating the importance of vlsE antigenic variation during the pathogen's enzootic cycle are nonexistent. Our long-term goals are to determine any potential modulatory effects by VlsE on the host immune response. The objective of this application is to determine whether the escape of B. burgdorferi surface antigens from immune surveillance in the host reservoir requires VlsE and host immune molecules. Based on preliminary findings, our central hypothesis is that VlsE mediates immune evasion of the BBF01 lipoprotein from antibodies of the reservoir host using a process that requires IgM. The rationale for the proposed research is that identifying the presence and molecular details of a VlsE-promoted protection system in B. burgdorferi would significantly advance our knowledge how this pathogen evades with the host immune system. Thus, the proposed research is relevant to that part of NIH's mission that pertains to developing fundamental knowledge that will potentially help to reduce the burdens of human illness and disability. Guided by preliminary findings, our hypothesis will be tested by pursuing three specific aims: 1) Demonstrate a VlsE requirement for B. burgdorferi surface antigen immune avoidance during reinfection of the natural reservoir host; 2) Determine a VlsE requirement for BBF01 evasion from host antibodies; and 3) Determine a requirement for IgM antibodies in VlsE-mediated immune evasion. Under the first aim, VlsE-naive Peromyscus mice will be challenged with either tick-derived mutant or wild type Borrelia to look for a capacity for host reinfection. Under the second aim, mice immunized with BBF01 antisera will be challenged with either tick-derived wild type or VlsE/BBF01-deficient clones to determine if the presence of VlsE prevents BBF01 from being recognized by host antibodies. Finally, the third aim will utilize Rag-/- mice infected with either mutant or wild type Borrelia i order to assay for an IgM requirement for VlsE-mediated antibody evasion. The proposed work is innovative, because it involves reinfection of reservoir mice with an active humoral response to B. burgdorferi in order to address the question of VlsE-promoted protection, and utilizes various vlsE mutant B. burgdorferi clones for infection of both the tick vector and host reservoir. This approach will likely provide more useful knowledge in developing strategies to prevent and treat Lyme disease in humans.